This invention relates to improvements in the preparation of dicarboxylic acid anhydrides by the vapor phase oxidation of hydrocarbons and more particularly relates to improvements in the process for the catalytic oxidation of hydrocarbons to dicarboxylic anhydrides in the presence of a vanadium -- molybdenum -- oxygen catalyst.
It has recently been discovered that high yields of dicarboxylic anhydrides may be obtained by oxidizing hydrocarbons in the vapor phase in contact with a vanadium -- molybdenum -- oxygen catalyst. Although high yields of dicarboxylic anhydrides have been obtained by such processes, it has been found that the yield of product diminishes with time. It is an advantage of this invention to provide a method whereby the useful life of the catalyst bed may be extended by in situ treatment and whereby the high yields may be maintained. It has been found as part of the present invention that one reason for the decrease in yield is the progressive non-selectivity of a portion of the catalyst particles with use and a decrease in the anhydride yield of the process. It is also a feature of this invention to selectively deactivated catalyst particles without impairing the activity of the remaining catalyst particles. This type of phosphorus treatment is readily distinguished from processes such as those disclosed in U.S. Pat. Nos. 3,296,282 and 3,474,041 and British specification 1,291,354 wherein phosphorus is added to replenish phosphorus which is lost during the use of the catalyst and phosphorus added there to actually regenerates the catalyst by replacing lost phosphorus.